The objectives of this investigation are:
* To understand the capabilities and limitations of using thermocouples to measure temperature.
* To determine the temperature profile in a furnace.
The results that were obtained in the experiment were fairly accurate and precise. The RTD used in the experiment was however more accurate compared to the thermocouples. Even though the thermocouples gave temperature readings to at least a count of 1ï¿½C, the RTD measured the temperature to an accuracy of 0.1ï¿½C. By looking at table 1(in Appendix B), you can see that for each system, the RTD measurements lies in the range of that measure for the thermocouples. For instance, for the distilled water and crushed ice, the type k thermocouple showed that the temperature was between 0 and 1ï¿½C; the RTD measurements showed that the temperature was 0.02ï¿½C, which is between 0 and 1ï¿½C and is consistent with the readings of the thermocouple.
Even though the RTD is more accurate, any of the other devices can be used in situations where a high degree of accuracy is not needed. This is because they are equally precise and give temperature readings that fall within the same range. During the experiment, certain factors had to be taken into consideration when using the RTD’s and thermocouples. Firstly, the temperature measurements by the RTD had a small error of 0.01ï¿½C.this error was caused by the introduction of a small protection sheath which was around the RTD.
This error is inherent with the RTD and is hard to avoid as a protection sheath is necessary. An error of 0.01ï¿½C is very small and therefore should not have any major effect on the experiment. Also, this error can be calculated and an accurate answer can be obtained after subtracting the error from the final answer.
Secondly, it is worth mentioning that the length of the cable used to measure the temperature will have an effect on the results due to there being a small loss in potential as electricity travels through the cables. This loss of potential is rather small and can therefore be ignored for most measurement purposes if the length of the cable is less than 25 meters. If the length of the cable used to measure the temperature happens to exceed 25m, then a correction factor should be incorporated into the temperature measured or an amplifier can be used. An amplifier should increase the energy of the signal generated by the probe and should compensate for transmission losses.
An important characteristic of a temperature sensor that should be considered before an experiment is its tolerance. ”Tolerance” in terms of temperature measurements, is an indicator for the sensitivity or accuracy of a temperature measuring device. Since it is impossible to produce a device that is of100% accuracy, it is therefore important to know the tolerance of the sensors. A device with a high tolerance tends not to be very accurate, whereas a device with a low tolerance shows a high degree of accuracy.
In the experiment, we (as a group) observed that switching the thermocouples around caused the EMF reading to double. This implied that whiles using the thermocouples, you have to make sure that the electrodes are connected in the correct positions or else you will obtain a large error in your readings. The cause of the variation of voltage with the position in which the electrodes are connected is due to the Seebeck Effect. This effect states that different types of metals respond differently to temperature differences and therefore produce different potential differences.
The second part of the experiment was to observe the variation of temperature with position near a furnace. Figure 1 shows that as the thermocouple is moved close to the furnace, the temperature first increases gradually and then remains constant and then finally decreases slightly at positions 7 to 10.
Figure1: Graph showing the variation of temperature against position.
With the support of the results obtained from the experiment, one can reach a conclusion that the temperature depends on a wide range of factors. One can state that the temperature depends on the length of wire used, the type of device used and also the tolerance of the device used. Also, one can conclude that the temperature also varies with temperature. If this experiment was to be carried out again, it should be recommended that variation of temperature with position near a furnace should done using different types of thermocouples.
1) TEMPERATURE MEASUREMENT(TM 1) -(handout)
Appendix A: Experimental Programme
Measuring Temperature with Thermocouples
1) Make up a basic thermocouple circuit using the exposed junction sensor, K1 (type K chromel-alumel), by fitting the plug to socket RK. The configuration is equivalent to Figure XXX without extension leads.
Switch the DVM Selector to position 1 and measure the thermocouples EMF. Record your results and compare them with values from standard tables.
(Note: probes must be fully inserted into the 0ï¿½C and 100ï¿½C tubes during these tests. Record the thermometer reading at steam point.)
2) Disconnect the thermocouple K1 from RK and connect the green lead KL and connect K1 to this lead.
Repeat the measurement procedure for readings at 0oC and 100oC.
3) Reverse the contact connections for connector KL1 and repeat the reading at 0ï¿½C and 100ï¿½C.
4) Remove lead KL and connect lead EL (purple lead).
Connect K1 to this lead and repeat readings at 0oC and 100oC.
5) A digital thermometer DTJ1 and thermocouple probes J1 and T1 are provided. Plug thermocouple J1 into digital thermometer DTJ1 socket SKJ1.
Check the calibration of probe J1 at 0 ï¿½C and 100ï¿½C.
Remove probe J1 from DTJ1 and replace with probe T1.
Check calibration of probe T1 at 0 ï¿½C and 100ï¿½C
Remove probe T1 and replace J1.
Measure the temperature at the input connector on DTJ1.
In the experiment, the temperatures of these systems are going to be measured:
I. Distilled water and crushed ice
II. Tap water and ice
III. Boiling water
IV. Swapping polarity
V. Extension cable
The temperature of the first three systems is going to be measured twice, firstly by using a RTD (Resistance Temperature Detector) and then a type k thermocouple. This therefore means that there is going to be a total of eight investigations to be carried out.